Acid-salt metathetic process

ABSTRACT

A cation exchange membrane mediated acid-salt metathetic process. An aqueous salt solution is placed into a first compartment on one side of the membrane together with an organic amine extractant of limited water miscibility, and an aqueous acid solution is placed into a second compartment on the other side of the membrane. Product acid forms in the first compartment and collects in the organic amine extractant from where it is recovered. The process is particularly suitable for the recovery of a carboxylic acid from its salt.

FIELD OF THE INVENTION

The present invention is in the field of acid-salt metathetic processesin aqueous solution of the kind that are described by the equation I

    MX+HY⃡MY+HX                                    I

wherein M stands for a metal cation, H for a proton and X and Y for twodifferent anions.

In the above equation I, the left hand side members MX and HYconventionally denote the starting materials, i.e. the reactants, andthe right hand side members MY and HX the products. In keeping with thatconvention, when the reaction proceeds from left to right it is referredto as a forward reaction and when it proceeds from right to left it isreferred to as a backward reaction.

BACKGROUND OF THE INVENTION

Metathetic processes of the kind specified occur in aqueous solutionunder conditions in which the forward reaction is favored. Where one ofthe products MY and HX is water insoluble or volatile and accordinglyprecipitates or evaporates from the solution, the equilibrium of thereaction is constantly shifted from left to right leading to acontinuous forward reaction. Depending on whether the desired product isthe salt MY or the acid HX or possibly the two of them, the reactionsolution and/or the precipitate must be worked up for the recovery ofthe desired product therefrom. Where, however, both the product MY andHX are water soluble, a steady state will be reached after a while inwhich the forward and backward reactions are in equilibrium and somekind of intervention is accordingly required for inducing continuationof the forward reaction. In the following, a metathetic reaction of thekind specified in which one of the two products is of limited watersolubility or higher volatility and accordingly separates from theaqueous reaction mixture in the course of the reaction, will be referredto as spontaneous forward reaction while a reaction in which bothproducts are water soluble and intervention is required for shifting theequilibrium from left to right and thereby induce continuation of theforward reaction, will be referred to as an induced forward reaction.

A typical example of metathetic processes with spontaneous forwardreactions are the so-called wet process production of phosphoric acid byreaction of calcium phosphate with sulfuric acid, and the recovery ofcitric acid from a fermentation liquor by the so-calledliming/acidulation process, which may be described, respectively, by thefollowing Equations II and III

    Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3 PO.sub.4II

    Ca.sub.3 (Cit).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3 PO.sub.4III

Typical examples for metathetic processes with induced forward reactionsare production of the multi-nutrient fertilizer potassium nitrate frompotassium chloride and nitric acid, in which the product of interest isa salt; and the conversion of ammonium lactate, which is a directproduct of lactic acid fermentation, into free lactic acid by reactionwith sulfuric acid, the product of interest here being an acid. Thesetwo processes are described, respectively, by the following equations IVand V

    KCl+HNO.sub.3 =KNO.sub.3 +HCl                              IV

    2NH.sub.4 La+H.sub.2 SO.sub.4 =(NH.sub.4).sub.2 SO.sub.4 +2HLaV

where La is lactate.

In reaction IV the main product is KNO₃ and in reaction V the product ofinterest is HLa. In both reactions, the products are water soluble whichmakes it necessary to induce the forward reaction.

Solvent extraction is commonly applied for the inducement of forwardreactions by product separation in metathetic processes. Alkanols,ethers, esters, ketones and other oxygen-carrying, water-immisciblecompounds are well known acid extractants operating through salvation ofthe acid (solvating extractants). Acid binding in such extractingoperations is relatively weak and as a result, the solvating extractantsare effective only at relatively high acid activities, and by themselvesare, as a rule, not capable of providing the driving force required forthe inducement of the forward reaction. While the reaction of potassiumchloride with sulfuric acid to form potassium sulfate and hydrochlorideacid can be facilitated by acid extraction with alkanols, at the aciditylevels required for efficient extraction, the acidic salt KHSO₄ isformed rather than K₂ SO₄. In addition, Cl⁻ /SO₄ ²⁻ extractionselectively is low which entails that for effective separation of HClfrom KHSO₄ many extraction stages are required. Quite generally, due tolow binding energy, solvating extractants are not effective forinducement of the displacement of a strong acid by a weak one.

Amine based extractants are much stronger acid binders and thuseffective also at low acidities. They are accordingly capable ofproviding acids, the driving force for metathetic processes in whichweak acids react with salts of strong acids, and they are accordinglywidely used for the withdrawal of product acids and the inducement ofthe forward reaction in a metathetic process.

Amine based extractants have, as a rule, a high anion/anion selectivityand thus provide for good separation from each other of the anions inthe system and thereby for higher yields and higher purities of theproducts. This high selectivity may, however, become counter productiveand induce backward reactions in cases where the starting acid HY ispreferentially extracted and this indeed is the case in the processaccording to equation IV above in that from a solution containing theions Cl⁻, NO₃ ⁻, H⁺ and K⁺, HNO₃ is extracted preferentially by aminebased extractants and it is for this reason that amine based extractantscannot be used in the case of equation IV and less attractive solventextractants or other means are required. Thus, in industrial processesfor KNO₃ production, the by-product HCl is removed through chemicalconversion. In one process KCl and HNO₃ are reacted at temperature,concentration and acidity at which oxidation/reduction reactions takeplace whereby chlorine, nitrogen oxides and other products are formed.Complex separations procedures, NOx conversion to nitric acid andoperation at highly corrosive conditions, all of which are required forinducing the forward reaction, result in low profitability.

KNO₃ production from KCl and NHO₃ can also be mediated by ionexchangers. Passage of a KCl solution through a cation exchanger in itsacid form results in loading the resin with K⁺ ions and formation of anHCl solution. The cation exchanger is then eluted with an HNO₃ solutionto produce KNO₃ and regenerate the cation exchanger. Alternatively, itis possible to transfer KCl solution through NO₃ ⁻ loaded anionexchanger which is then regenerated by HNO₃.

Likewise, solvent extraction with an amine based extractant also inducesbackward reaction in case of equation V above in that, from an aqueoussolution containing La⁻, SO₄ ⁻, HSO₄ ⁻, H⁺ and NH₄ ⁺, HSO₄ is extractedpreferentially whereby the backward reaction is induced. Solvatingextractants have the required selectivity and extract lactic acid fromthe solution without, however, yielding the advantages afforded by aminebased extractants. Accordingly, it has been proposed to recover lacticacid from a fermentation broth by processes other than a metatheticreaction. Thus, EP 0517242 (Mantovani et al.) describes a process bywhich the lactate values of a nearly neutral lactic acid fermentationliquor are first separated by means of a carbonate loaded anionexchanger, which is then eluted by ammonium carbonate to form an aqueousammonium lactate and an ammonium carbonate solution. The ammoniumcarbonate in solution is decomposed thermally and the ammonium lactateis passed through a cation exchanger in its acid form to yield a lacticacid solution and an ammonium loaded cation exchanger, which latter isregenerated by a mineral acid solution such as aqueous HCl. This processthus involves two ion exchange operations, the first of which providesfor purification while the second mediates the metathetic reaction ofequation VI

    NH.sub.4 La+HCl=NH.sub.4 Cl+HLa                            VI

In order to avoid product or by-product contamination and low yield,large volumes of ion exchanger and marked dilution are applied in boththe above ion exchange operation. Further dilution is imposed byunavoidable washing of the ion exchanger resin after each stage, all ofwhich results in a dilute product solution. It is thus evident thatthese and similar processes are cumbersome and not practical forindustrial application.

It is the object of the present invention to provide in an acid-saltmetathetic process, a new method for inducing forward reactions.

SUMMARY OF THE INVENTION

The invention makes use of a known technique by which metatheticacid-salt reactions are performed indirectly with cation exchangemediation. Thus Korngold and Vofsi describe in "Desalination, 84, 123(1991) a process for the removal of salts from water, comprising in afirst stage interaction of a salt contained in a first aqueous solutionwithin a first compartment with an acid contained in a second aqueoussolution within a second compartment, which compartments are separatedfrom each other by a cation exchanger membrane. In the course of theprocess the cation of the salt transfers via the membrane from the firstto the second solution and protons are transferred in the oppositedirection to the first compartment. As a result, an acid is formed inthe salt compartment and a salt in the acid compartment.

U.S. Pat. No. 4,818,409 (Puetter et al.) describes a process forobtaining an aqueous solution of a water-soluble organic acid. Inaccordance with that disclosure, an aqueous solution of a salt of awater-soluble organic acid having a pK_(a) value>2 is passed through afirst compartment on one side of a cation exchanger membrane and anaqueous solution of a mineral acid is passed through a secondcompartment on the other side of the membrane.

In both these processes, the driving force for the indirect reaction isprovided by the reactants themselves without any extraneous inducement.Thus, in the Korngold and Vofsi process the driving force for theindirect reaction and thus the prerequisite for its occurrence, is ahigh proton concentration, i.e. a high acidity. This prerequisite is,for example, not met in the recovery of an organic acid from a startingsolution of its salt, e.g. the recovery of lactic acid from an ammoniumlactate broth obtained by lactic acid fermentation processes, where itis preferred to use weak acids. In the Puetter et al. process, thedriving force is the high basicity of the organic acid anion and fordriving the reaction to completion a strong mineral acid is againrequired and a relatively weak acid such as, for example, acetic acid orphosphoric acid will in many cases not be effective. The unsuitabilityof phosphoric acid presents a serious drawback since phosphate saltswhich can be used as fertilizers, are preferred by-products ofcarboxylic acid production.

In accordance with the invention it has surprisingly been found that acombination of cation exchanger membrane mediation with solventextraction of product acid by means of an organic amine extractant, isvery effective for the inducement of forward reactions in acid-saltmetathetic reactions. It has further been found that as a result of saidcombination any undesired and counter-productive anion/anion extractionselectivity is avoided.

The invention thus provides in a cation exchanger membrane mediatedacid-salt metathetic process in which the starting materials andproducts are water-soluble and in which an aqueous solution of astarting salt is charged into a first compartment on one side of thecation exchanger membrane and an aqueous solution of a starting acid ischarged into a second compartment on the other side of the membrane, theimprovement by which the said first compartment is further charged withan organic amine based extractant of limited water miscibility whereby abinary liquid system is formed in said first compartment comprisingaqueous and organic phases, and the organic phase is separatelywithdrawn from said first compartment after a desired residence time forthe recovery of a product acid therefrom.

Thus, as distinct from the prior art where the reaction between thestarting salt MX and the starting acid HY is direct, in accordance withthe invention, it is indirect in being mediated by the cation exchangermembrane with the driving force being provided by the amine basedextractant.

The process according to the invention may be carried out batch-wise orcontinuously. In accordance with one embodiment of a continuous mode ofoperation, a multi-cell battery is used with alternating first andsecond compartments, said aqueous solution of a starting salt and saidorganic amine based extractant being continuously passed through allsaid first compartments and said aqueous solution of a starting acidbeing continuously passed through all said second compartments.

In accordance with another embodiment of the continuous mode, there isused a system of hollow fibrous cation exchanger membranes containedwithin a suitable vessel. The aqueous solution of a starting salt andthe amine based extractant are mixed in the vessel while said aqueoussolution of a starting acid is flown through the hollow fibrousmembranes.

In the performance of the process according to the invention, thecations of the starting salt are transferred from a first to a secondcompartment, protons are transferred from a second to a firstcompartment, the product acid is continuously withdrawn into saidorganic amine extractant of limited water miscibility within a firstcompartment and in this way there is induced the forward reaction in thecation exchanger membrane mediated acid-salt metathetic reaction. Asmentioned, in this process the amine extractant provides the drivingforce for the performance of the metathetic reaction and makes possiblethe occurrence of reactions which otherwise would not have beenfeasible, e.g. reactions between salts and weak acids such as carbonicacid, phosphoric acid, acetic acid and the like. Moreover, unlike in thecase of direct reaction in which the anion selectivity of an amineextractant may be counter productive and favor the backward reaction, inthe presence of a cation exchanger membrane as taught by the presentinvention, by which the reaction is rendered indirect, any anionselectivity of the organic amine extractant is of no consequence anddoes not influence the direction of the reaction.

The use of an organic amine extractant also provides for purification ofthe product acid from impurities present in the aqueous solution of thestarting salt. For example, when applying the invention to the recoveryof lactic acid from a nearly neutral fermentation liquor the productlactic acid recovered from the separately withdrawn organic phase, ispractically free of hydrophilic neutral impurities.

In the implementation of the process according to the invention, productacid is recovered from the organic phase that is separately withdrawnfrom a first compartment, and the regenerated organic amine extractantis recycled.

If desired, a product salt may be recovered from the aqueous solutionwithdrawn from a second compartment, either as a useful by-product wherethe acid recovered from the organic phase withdrawn from a firstcompartment is the main product, or as the main product of the process.

Organic amines suitable for use as constituents of extractants areprimary, secondary and tertiary amines having a total of at least 18carbon atoms, and they may be used by themselves or optionally togetherwith diluents such as non-polar hydrocarbons, and/or together with polaror protic extraction enhancers and/or together with water-immiscibleorganic acids.

Recovery of the product acid from the organic phase withdrawn from afirst compartment can be effected by conventional means. Thus, where theproduct acid is volatile such as in the case of hydrochloric or aceticacid, and the various extractant components have a sufficiently highboiling point and do not form azeotropes with the product acid, the acidcan be recovered by distillation.

In case the product acid in the process according to the invention isnot volatile and notably in case of non-volatile mineral acids, theorganic amine of the extractant is preferably a so-called reversibleamine, i.e. an amine which readily releases its acid content into water,and the product acid is withdrawn from the organic phase byback-extraction with water. The so-called reversible amines arerelatively weak amines such as, for example, tris-2-ethylhexyl amine,and various aniline derivatives. Another family of suitable extractantsare various long-chain organic amines in combination with an organicacid and a hydrocarbon diluent, as described in U.S. Pat. No. 4,291,007.

Also carboxylic acids obtained as products can be recovered from theorganic amine extract withdrawn from a first compartment byback-extraction with water. It is, for example, possible to employ themethod disclosed in U.S. Pat. No. 4,275,234 according to whichback-extraction of the amine extract with water is effected at atemperature higher than the temperature at which the acid is extractedinto the extractant, i.e. the temperature of the metathetic reaction.Back-extraction can be further enhanced by adding extractant suppressorssuch as non-polar diluents to the acid-loaded organic phase (extract)prior to back-extraction and/or by removing from the extract prior toback-extraction enhancers such as alkanols.

Conventional, commercially available cation exchanger membranes such asNEOSEPTA™, SELEMION™ and NAFION™ are suitable for the purposes of thepresent invention. Typical cation exchanger membranes carry sulfonicgroups, such as sulfonated polystyrene interpolymers with neutralpolymers; sulfochlorinated polyethylene; sulfonated polysulfone,sulfonic perfluorinated polyolefins; etc.

The invention is highly suitable for metathetic processes in variousfields of chemical and biochemical technologies such as recycling offermentation products, fertilizer production and waste treatment, asshown by the following.

Introduction of an amine based extractant and an aqueous KCl solution onone hand and an aqueous HNO₃ solution on the other to a battery ofalternating first and second compartments separated by cation exchangermembranes, results in the formation of an HCl loaded extractant in thefirst compartments and an aqueous KNO₃ solution in the secondcompartments. Similarly, aqueous ammonium lactate and amine basedextractant in first compartments and phosphoric, sulfuric or nitric acidin second compartments results in lactic acid loaded extractant and inan aqueous solution of an ammonium salt suitable for use as fertilizer.Fluorosilic acid obtained as a by-product in wet-process phosphoric acidis an attractive source of fluoride for HF production. Reaction withammonia to form ammonium fluoride and silicic acid is a straightforwardreaction but recovery of HF values from ammonium fluoride solutions byknown processes was found too complicated to be economic. The currentinvention provides for an economic recovery through conversion of NH₄ Finto HF and an ammonium salt such as phosphate, sulfate or nitrate.Another example of application is the conversion of ZnCl₂ obtained inwaste streams of metal surface treatment, into ZnSO₄ which latter issuitable for Zn production through electrowinning.

Solvent extraction is highly suitable for recovery of acids fromsolutions comprising acids and their salts. Thus, pure phosphoric acidcan be recovered from agriculture grade phosphoric acid throughextraction by reversible extractants. However, due to presence ofcations in the solution (namely Fe⁺³, Al⁺³ and Mg⁺²), up to one third ofthe phosphate values are not taken up by the extractant and remain inthe raffinate as monophosphate. According to the invention, the cationcontaining acid or the extraction raffinate is fed to first compartmentstogether with an amine-based extractant, and a low-cost acid (HCl, H₂SO₄) is fed to second compartments of a battery of alternatingcompartments separated by cation exchanges membranes, wherebypractically all residual phosphate values are recovered.

Acid containing wastes are obtained in many industries such ashydrometallurgic processes and surface treatments. Recovery of acidvalues through solvent extraction reduce waste treatment costs ascompared to neutralization, but does not eliminate waste management asin most cases the remaining salt solution cannot be disposed of. Theinvention provides for the recovery of the acid values combined withconversion of a waste salt to another one, which is more suitable forfurther treatment. Thus the acidic waste stream of TiO₂ pigmentproduction, containing H₂ SO₄ and FeSO₄, can be treated for the recoveryof most of the sulfate values and for conversion of the FeSO₄ to FeCl₂.

An important application of the present invention is in the field ofcitric acid recovery from fermentation liquors. The traditionalliming/acidulation process has many drawbacks. Solvent extractionprocesses, applying amine based extractant, such as described in U.S.Pat. Nos. 3,944,606 and 4,275,234 have the serious limitation ofrecovering only free acid values and are therefore impractical for therecovery of citric acid from broths obtained by fermentation processesbased on molasses as sole carbohydrate source or as an additive to purecarbohydrate. Such nutrients yield, among others, citrate salts asprimary fermentation products, which do not report to the extractant.Application of the invention provides for extraction of practically allcitric acid values from cation containing fermentation liquors and fromother contaminated citric acid streams, with no formation of gypsum asan added benefit.

Hydrophilic neutral impurities present in the reagent salt solution in afirst compartment will not follow the cation to the product salt in asecond compartment across the cation exchanger membrane, and at the sametime the selectivity of the amine based extractant provides for theextraction of the product acid in a pure form. As a result, hydrophilicneutral impurities will not report to either of the products and willremain in the aqueous solution left behind in a first compartment afterseparate withdrawal of the extractant. In this way, product acids andsalts are obtained in pure form which is of particular importance whenone of the products is a carboxylic acid recovered from a fermentationbroth.

Acid extraction into the organic extractant in the first compartment andsalt formation in the second compartment deplete the feed salt solutionof ions. Accordingly, osmotic pressure will drive water from the firstinto the second compartment which enhances the extraction of the acid inthe first compartment.

Where the product salt is the product of interest the selection of thestarting acid is governed by the nature of the salt to be formed anddisposal of the by-product acid should be taken into consideration.Typical examples of starting acids used in accordance with the inventionare sulfuric, nitric and hydrochloric acid. CO₂ and SO₂ may also be usedas acid sources. A solution comprising an acid and a salt, e.g. theproduct of cation exchanger regeneration is also suitable as a reagentacid solution. This aspect is of considerable practical importance sinceit makes possible to use excess acid for the regeneration of a cationexchanger and use the regeneration effluent liquor to advantage.

Where the product acid is the product of interest, the selection ofreagent salts will be governed by economic production considerations.Product acids in the processes according to the invention include aminoacids and for their production amino acid salts may serve as reagentsalts.

For better understanding the invention will now be illustrated by thefollowing examples to which it is not limited.

EXAMPLE 1

An apparatus was used holding two 30 ml. compartments separated by aNeosepta CN-1™ (Tokoyama Soda) cation exchanger membrane with an activearea of 2 cm². The membrane was supplied within an NaCl solution and wastherefore rinsed with water prior to its mounting in the apparatus.

The first cell of the apparatus was loaded with 20 gr. of an aqueous 1mol/kg KCl solution and 5 gr of an organic extractant comprising 1.075mol/kg of trilaurylamine (Alanine 304™, Henkel), 5% by weight of octanoland kerosene as diluent. The second compartment was loaded with 20 gr ofan aqueous solution containing 1 mol/kg of HNO₃. The apparatus was keptin a mechanical shaker for 3 days following which the organic phase wasseparately withdrawn from the first compartment and analyzed by separateproton and chloride determination which showed that it contained 0.38mol/kg of HCl and practically no HNO₃. Analysis of the secondcompartment showed that it contained 1.9 mmol of KNO₃. The withdrawnextractant was regenerated by contact with a solution of ammonia wherebyHCl was removed.

In a comparative experiment 5 gr of extractant of the same compositionwas equilibrated in direct contact in a separatory funnel with anaqueous solution containing 1 mol/kg KCl and 1 mol/kg HNO₃. Most of theacid found in the organic phase was HNO₃.

EXAMPLE 2

The same apparatus and extractant were used as in Example 1. The aqueousstarting salt solution introduced into the first compartment contained 1mol/kg monosodium citrate and the aqueous starting acid solutionintroduced into the second compartment contained 1 mol/kg HCl. After 3days in a mechanical shaker the extractant contained 0.7 mol/kg citricacid with only traces of HCl. The extractant phase was back-extracted by5 successive equilibrations at 80° C. with water at anorganic-to-aqueous ratio of 1:1. Analysis of the combined aqueoussolutions show that practically all the citric acid was back-extracted.

In a comparative experiment an extractant of the same composition asabove was introduced into the first compartment of the apparatus and anaqueous solution containing monosodium citrate and hydrochloric acid ina concentration of 1 mol/kg each were introduced in to t he secondcompartment. After 3 days of shaking the extractant remained was nearlyfree of acid.

EXAMPLE 3

The equipment, extractant and procedure were similar as in the previousexamples. The aqueous starting, salt solution contained 1 mol/kg sodiumlactate and the aqueous starting acid solution contained 1 mol/kg aceticacid. After 3 days the extractant was loaded with 0.25 mol/kg of lacticacid which was recovered by back-extraction with water at 80° C.

In a comparative experiment 5 gr of extractant and 20 gr of an aqueoussolution containing 1 mol/kg of sodium lactate and 1 mol/kg of aceticacid were contacted in a separatory funnel. Exctractant loading, withlactic acid in equilibrium was 0.04 mol/kg.

In another comparative experiment the above extractant and water wereintroduced into the first compartment of the apparatus and an aqueoussolution containing 1 mol/kg of acetic acid into the second compartment.Acid concentration in the extractant after 3 days of shaking was lessthan 0.04 mol/kg.

I claim:
 1. In a cation exchanger membrane mediated acid-salt metatheticprocess in which the reactants and products are water-soluble and inwhich an aqueous solution of a starting salt is charged into a firstcompartment on one side of the cation exchanger membrane and an aqueoussolution of a starting acid is charged into a second compartment on theother side of the membrane, the improvement by which the said firstcompartment is further charged with an organic amine extractant oflimited water miscibility whereby a binary liquid system is formed insaid first compartment comprising aqueous and organic phases, and theorganic phase is separately withdrawn from said first compartment aftera desired residence time for the recovery of a product acid therefrom.2. The process of claim 1 carried out batch-wise.
 3. The process ofclaim 1 carried out continuously.
 4. The process of claim 3, carried outin a multi-cell battery with alternating first and second compartments,said aqueous solution of a starting salt and said organic amineextractant being continuously passed through all first compartments andsaid aqueous solution of a starting acid being continuously passedthrough said second compartments.
 5. The process of claim 3, whereinthere is used a system of hollow fibrous cation exchanger membranescontained within a suitable vessel, said aqueous solution of a startingsalt and said amine based extractant being mixed in the vessel and saidaqueous solution of a starting acid being flown through the hollowfibrous membranes.
 6. The process of claim 1, serving for the conversionof a carboxylic acid salt into free carboxylic acid.
 7. The process ofclaim 6, serving for the recovery of lactic acid from a fermentationbroth.
 8. The process of claim 6, serving for the recovery of citricacid from a fermentation broth.
 9. The process of claim 6, serving forthe conversion of an amino acid salt into free amino acid.